1. Field of the Invention
The present invention relates to underground tanks and, more particularly, to an improved underground tank design incorporating a toroidal shape for improved strength.
2. Description of the Background
Underground storage tanks have long been used for storing various fluids, including gasoline, water, or other chemicals. Such tanks must withstand pressure from the surrounding soil, elevated water tables, and the tank contents. In addition, they must be resistant to the corrosive effects of their contents and the surrounding soil. Thus, the structure of underground storage tanks must be robust and liquid tight, or leakage will occur.
Underground water storage tanks and septic tanks were traditionally constructed of masonry materials such as stone and brick. Since masonry construction is very labor intensive and difficult to make liquid-tight, it was largely replaced by site-cast or pre-cast concrete construction. Underground fuel storage tanks were traditionally constructed of steel.
Both concrete and steel are susceptible to corrosion, especially where tanks are buried in wet soils. They are also very heavy, complicating transport and installation. These problems spurred the development of corrosion-resistant tanks constructed of fiberglass reinforced plastic (FRP), polypropylene (PP), and polyethylene (PE).
Xerxes Corporation, a leader in fiberglass reinforced plastics, and its predecessor, introduced FRP underground storage tanks in the 1970's, along with Owens Corning Fiberglass. These tanks have been proven reliable and are widely used for fuel storage and other applications where strength is primary. However, FRP tanks are expensive and are prone to damage in shipping and installation. In addition, inlet, outlet, and access connections to FRP tanks can be problematic and expensive.
In comparison with FRP tanks, rotationally molded PE tanks are inexpensive to produce and are resistant to damage in transport and installation because they flex under impact. A variety of reliable connections can be formed into the rotational molds at minimal additional cost. For these reasons, PE tanks are widely used for small-scale water storage and on-site waste treatment applications.
Since PE is a relatively weak material that creeps under constant pressure, many designs have been proposed to increase the strength of underground PE tanks. Spherical shapes are very strong but do not pack well for transport and can be difficult to handle. In addition, to achieve significant capacity, spherical tanks must have a large diameter, which necessitates a deep excavation. For these reasons, spherical tanks are typically limited to 500 gallon capacity.
Flattened spheres and horizontal cylinders of PE offer larger capacities without the need for deep excavation, but at a significant loss of strength. To compensate, such shapes are typically strengthened with reinforcing ribs. For example, U.S. Pat. No. 4,359,167 to Fouss et al. shows a ribbed flattened spherical storage tank having a plurality of integrally molded reinforcing ribs. Ribs greatly strengthen tank walls against the crushing forces of hydrostatic pressure, but add considerable material cost. Ribs also trap fluids and air, reducing the effective volume of the tank. Ribs also create possibility of accordian-like folding under stress.
Other efforts have been directed toward internally reinforcing tanks to increase strength. For example, U.S. Patent Application Publication U.S. 2004/0011786A1 by Wade shows a molded cylindrical tank with pass-through internal support columns to prevent crushing. However, tanks with multiple internal columns have reduced volume and can be difficult to mold.
Still other efforts have utilized toroidal or “doughnut” shapes that exhibit sphere-like strength characteristics. For example, U.S. Pat. No. 4,615,452 to Lederer et al. discloses compound toroidal tanks with at least two stacked concentric toroidal cells sharing a common axis, with a partition and references prior art showing an unpartitioned torus. These conventional toroidal configurations offer increased strength but suffer from low storage capacity and high material cost in relation to their overall dimensions.
The present inventor recognized great potential of the toroidal shape in the context of an underground storage tank, especially a rotationally-molded PE tank, and has now developed an improved underground storage tank configuration and a number of tank embodiments incorporating the base configuration.